Geothermal facility with thermal recharging of the subsoil

ABSTRACT

Disclosed is a facility for transferring thermal energy of geothermal origin, for example to contribute to the heating of premises, this facility having the feature of including a heat exchanger placed in a water pipe ( 31 ). The principle of the facility is to extract the heat contained in the soil ( 1 ) in order to transfer it towards the required place (premises, offices, industrial processes, various equipment, etc.) and to recharge the soil ( 1 ) with heat extracted from a liquid used in public service, namely wastewater or drinking water, and/or used in industry.

This application concerns a heat-transfer facility using a heat pump, a geothermal source of heat energy, and at least one water line. It is used, in particular, for heating and/or air conditioning of buildings.

Geothermal power extracts the heat energy contained in the soil to produce heat or electricity. Geothermal power is commonly classified in three categories: high power (T>100° C.), low power (30° C.<T<100° C.), and very low power (T<30° C.).

High-power geothermal facilities allow for the production of electricity. Low-(direct use of deep underground layers of hot water) and very low-power (use via heat pumps) geothermal facilities are used to produce heat.

For very low-power geothermal power, one solution for recovering underground heat is to use geothermal probes, e.g., vertical probes. These probes are installed in a well several metres deep in which a coolant circulates in a closed circuit. The soil temperature at these depths is the result of an equilibrium between the exchanges with the atmosphere, the soil surface, and the geothermal flow originating from the depths. At 10 m of depth, it oscillates between 10 and 12° C.; thus, it is relatively stable. When a system draws calories from it, the equilibrium is re-established due to the exchanges with the surrounding soil.

However, if there is high demand, over the long term, the system can draw more calories than the soil can provide, as this equilibrium takes a relatively long time to establish. This results in a decrease in the temperature of the soil surrounding the probes, which results in degradation of the performance of the heat pumps over time, and an increase in power consumption. This thermal discharge of the soil may be quite substantial, in particular if there is a substantial imbalance, on a yearly average, between the energy extracted from the soil and the energy reinjected naturally into the soil around the pumps. The natural recharging of the soil by means of geothermal flow does not allow for the reattainment of a temperature equivalent to the initial level.

One solution is to resort to other heat sources to “recharge” the calories of the soil. Application FR 2 922 634 proposes a system using solar cells to “recharge” the calories of the soil. However, this method has disadvantages, as the solar cells are not a stable heat source, as their temperature depends essentially on solar irradiance. Furthermore, in the case of a reversible system, i.e., a system that can be used both to provide cold air to cool buildings, in particular in the summer, the system will “recharge” the underground calories when used as a source of cooling.

The invention thus more specifically has the objective of eliminating this disadvantage. To this end, the invention proposes a heat energy transfer system comprising:

-   -   at least one first heat exchanger arranged near a geothermal         power source,     -   at least one second heat exchanger arranged near a renewable         heat energy source comprising a connection to the water system,     -   one heat pump,     -   a coolant circuit linking the evaporator of the heat pump to the         first and second heat exchanger.

The invention concerns a geothermal heat energy transfer system, e.g., for the purpose of contributing to the heating of buildings, it comprises a heat exchanger arranged on a water line. The principle of the system is to draw the calories contained in the soil to transfer them to where they are needed (dwellings, offices, industrial processes, miscellaneous equipment, etc.), and to recharge the calories drawn from the soil using a liquid used in public utilities, wastewater, drinking water, and/or industrial water.

Advantageously, the water system connection can be a wastewater, industrial, or water conveyance system connection.

This water system connection may originate from a cooling network from an industrial process necessitating a cooling circuit.

Various exchanger technologies allow for the extraction of calories from water lines, in particular wastewater lines. The latter remain hot during climatic cold periods; they may thus serve as a heat source for the heat pumps. Various other liquids remain hot during the winter: potable water, industrial water, circulating in closed pipes, whether buried or not, may also be used as a heat source.

The advantage of this type of calorie source is that it is much more stable over time than solar cells. The temperature of wastewater, for example, varies between 13 and 17° C. in the winter, and 20 and 25° C. in the summer. This calorie source may be described as renewable, as the calories of the wastewater are a by-product of human activity that is otherwise wasted. Due to its relative persistence over time, it is entirely suited to serve as a “base” heat source for a building heating system. The estimated gain is approximately 30% compared to a system using geothermal energy alone.

Advantageously, the facility comprises a first mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the evaporator of the heat pump, then in the second heat exchanger, and then in the first heat exchanger.

In this configuration, the coolant loses calories during its passage through the evaporator of the heat pump, and is then reheated upon passing through the exchanger connected to the water system, and is then passed to the geothermal exchanger before returning to the evaporator. In this way, the fluid is recharged with calories originating from the water, e.g., wastewater, before drawing calories from the soil with the geothermal exchanger. Typically, the fluid can thus go, upon exiting the evaporator, from a temperature of approximately 1° C. to approximately 3° C. in the winter, or from approximately 7° C. to approximately 12° C. in the summer. This heating of the coolant reduces the underground calorie removal, improving the performance of the facility.

Advantageously, the facility comprises a second mode of operation, in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the first heat exchanger, and then in the second heat exchanger.

In this configuration, the coolant no longer passes through the evaporator of the heat pump, but rather circulates between the second exchanger coupled to the water system and the first exchanger in the soil. In this way, the facility recharges the calories of the soil. This is primarily a summer mode of operation, when the heat requirements are low or nil, and the water temperature, e.g., wastewater temperature, is relatively high.

The invention will be better understood based on the description below, which refers to the attached drawings, in which:

The sole FIGURE is a schematic representation of a heat transfer facility using geothermal probes and a water line.

Very low-power geothermal power is usually used to produce heat and cooling. This is generally done using a heat pump 8 that exchanges heat energy with the soil 1 via one or more geothermal probes 3, e.g., vertical probes.

It is known that a heat pump 8 is a thermodynamic machine suited to draw heat in a “cold source” environment, in which the temperature is below that required. It then transfers this energy to a coolant, in general hot water, but also sometimes air.

Heat energy is removed at low temperature at the level of the exchanger of the evaporator 7 of the heat pump 8. The heat is then transmitted at a higher temperature to the “hot source” at the level of the exchanger of the condenser 9 of the heat pump.

In the example shown, the vertical geothermal probes 3 each comprise a first exchanger 4, placed in a vertical well made in the soil 1 from the surface 2 of the soil. This well may be dozens of metres in depth. This exchanger 4 may then be sealed in the well with a grout (e.g., cement+bentonite). Several types of vertical geothermal probes are known, e.g., “U”, “double U”, or “coaxial”.

In the example shown, the heat pump 8 is schematically represented by a rectangular block comprising, on the one side, a subassembly comprising the exchanger of the evaporator 7 and, on the other, a subassembly comprising the exchanger of the condenser 9.

A coolant (generally water with the addition of antifreeze) circulates in a closed-loop circuit 40 by means of a circulation pump 6, which circuit 40 comprises:

-   -   first heat exchangers 4 coupled to a plurality of geothermal         probes 3,     -   a second heat exchanger 30 coupled to a wastewater line 31,     -   the evaporator 7 of the heat pump 8,     -   a first pipe 5 a, connecting the outlet of the first exchangers         4 and the inlet of the evaporator 7, a second pipe 5 b,         connecting the outlet of the evaporator 7 and the inlet of the         second exchanger 30, and a third pipe 5 c, connecting the outlet         of the second exchanger 30 coupled to the water line 31, and the         inlet of the first exchangers 4, coupled to geothermal probes 3;         a fourth pipe 5 d connects the first pipe 5 a to the second pipe         5 b, allowing the heat pump 8 to be bypassed. The circulation         pump 6 is mounted on the first pipe 5 a, but it could be mounted         on the second or third pipe; what is important is for it to be         able to drive the coolant in the two modes of operation of the         facility, i.e., that it be mounted on a part common to the         circuits implemented in these modes.

In a first mode of operation in heating mode, the gate 20 located on the first pipe 5 a is open, and the gate 21 located on the fourth pipe 5 d is closed. In this first mode of operation, the coolant circuits in a closed loop between the first exchangers 4, the evaporator 7 of the heat pump 8, and the second exchanger 30.

The coolant draws calories in the soil 1 due to the first exchangers 4, circulates in the first pipe 5 a to the evaporator 7 of the heat pump 8, loses these calories in the exchanger of the evaporator 7, takes the first pipe 5 b to the second exchanger 30, where it is heated, then circulates in the third pipe 5 c to return to the first exchangers 4 of the geothermal probes 3.

The coolant exchanges heat with the soil 1, which surrounds the geothermal probes 3, with the water line 31 and the refrigerant fluid of the heat pump 8.

If the temperature of the coolant circulating in the geothermal probe(s) 3 is less than the temperature of the soil 1 in contact with the probe(s) 3, the coolant is heated by removing heat from the soil 1 around the geothermal probes 3, thus cooling the soil 1 around these geothermal probes 3.

In this case, the facility removes calories from the soil 1, the heat pump 8 removes these calories from the coolant at the level of its evaporator 7, e.g., in order to heat a building and/or preheat domestic hot water. In order to minimise the cooling of the soil 1, the coolant takes the second exchanger 30 coupled to the wastewater line 31 so as to heat up before being directed to the geothermal probes 3.

Thus, the thermal discharge of the soil 1 is limited, thus limiting the decrease in temperature of the coolant circulating in the geothermal probes 3.

The heat removed by the heat pump 8 is then supplied to the hot source via the exchanger of the condenser 9. This heat is recovered by the fluid of the hot source, which circulates in the tubes 10 and 11 by means of a pump 12.

Outside of heating season, i.e., the time of year in which there is no need to heat, corresponding to the summer, the facility coupling the geothermal probes 3 to a wastewater line 31 operates to inject heat into the soil 1.

The facility can be placed in this mode of operation, e.g., depending on the external temperature (operation if the external temperature is greater than a set external temperature).

When this second mode of operation is activated, the gate 20 mounted on the first pipe 5 a is closed, and the second gate 21 mounted on the fourth pipe 5 d is open, i.e., the heat pump 8 is bypassed.

In this second mode of operation, the coolant circulates in a closed loop between the first exchangers 4 of the geothermal probes 3 and the second exchanger 30 connected to the water line 31 via the pump 6 mounted on the first pipe 5 a. The fluid is heated whilst traversing the second exchanging 30, then circulates in the third pipe 5 c, then in the first exchangers 4 of the geothermal probes 3, and returns to the second exchanger 30 by the pipes 5 d and 5 b. As the temperature of the coolant is greater than the temperature of the soil 1 surrounding the vertical geothermal probes 3, as it was heated by the second exchanger 30, the coolant loses heat to the soil 1 surrounding the geothermal probes 3.

This mode of operation thus allows thermal recharging of the soil 1 surrounding the geothermal probes 3 due to the heat energy supplied by the wastewater line 31. This thermal recharging allows for at least partial compensation of the thermal discharge of the soil 1 caused by the removal of heat from the soil 1 by the heat pump 8 by means of the geothermal probes 3 during the heating season. 

1. Heat energy transfer facility comprising: at least one first heat exchanger (4) arranged near a geothermal energy source (1), at least one second heat exchanger (30) arranged near a renewable heat energy source comprising a water system connection, a heat pump (8), a coolant circuit (40) connecting the evaporator (7) of the heat pump to the first and second heat exchanger.
 2. Heat energy transfer facility according to claim 1, wherein the water system connection is a wastewater system connection (31).
 3. Heat energy transfer facility according to claim 1, wherein the water system connection is a water conveyance system connection.
 4. Heat energy transfer facility according to claim 1, wherein the water system connection is an industrial water line.
 5. Heat energy transfer facility according to claim 1, wherein the water system connection originates from a refrigeration system of an industrial process necessitating a refrigeration circuit.
 6. Heat energy transfer facility according to claim 1, wherein it comprises a first mode of operation in which the coolant circuit (40) is a closed-loop circuit arranged such that the coolant circulates in the evaporator (7) of the heat pump (8), then in the at least one second heat exchanger (30), and then in the at least one first heat exchanger (4).
 7. Heat energy transfer facility according to claim 1, wherein it comprises a second mode of operation in which the coolant circuit (40) is a closed-loop circuit arranged such that the coolant circulates in the at least one heat exchanger (4), then in the at least one second heat exchanger (30).
 8. Heat energy transfer facility according to claim 2, wherein the heat energy transfer facility is constructed and arranged to operate in a first mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the evaporator of the heat pump, then in the at least one second heat exchanger, and then in the at least one first heat exchanger.
 9. Heat energy transfer facility according to claim 3, wherein the heat energy transfer facility is constructed and arranged to operate in a first mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the evaporator of the heat pump, then in the at least one second heat exchanger, and then in the at least one first heat exchanger.
 10. Heat energy transfer facility according to claim 4, wherein the heat energy transfer facility is constructed and arranged to operate in a first mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the evaporator of the heat pump, then in the at least one second heat exchanger, and then in the at least one first heat exchanger.
 11. Heat energy transfer facility according to claim 5, wherein the heat energy transfer facility is constructed and arranged to operate in a first mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the evaporator of the heat pump, then in the at least one second heat exchanger, and then in the at least one first heat exchanger.
 12. Heat energy transfer facility according to claim 2, wherein the heat energy transfer facility is constructed and arranged to operate in a second mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the at least one heat exchanger, then in the at least one second heat exchanger.
 13. Heat energy transfer facility according to claim 3, wherein the heat energy transfer facility is constructed and arranged to operate in a second mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the at least one heat exchanger, then in the at least one second heat exchanger.
 14. Heat energy transfer facility according to claim 4, wherein the heat energy transfer facility is constructed and arranged to operate in a second mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the at least one heat exchanger, then in the at least one second heat exchanger.
 15. Heat energy transfer facility according to claim 5, wherein the heat energy transfer facility is constructed and arranged to operate in a second mode of operation in which the coolant circuit is a closed-loop circuit arranged such that the coolant circulates in the at least one heat exchanger, then in the at least one second heat exchanger. 